1. Field of the Invention
The present invention relates to a control device for a spark-ignition engine and, in particular, to a control device for controlling conditions of combustion in individual cylinders to improve fuel economy and characteristics of emissions in a multicylinder engine.
2. Description of the Related Art
There is known a conventional technique for achieving an improvement in fuel economy by burning a lean mixture in individual cylinders of a spark-ignition engine at an air-fuel ratio larger than the stoichiometric air-fuel ratio. An example of this kind of technique is disclosed in Japanese Unexamined Patent Publication No. H10-274085. The technique of the Publication employs fuel injectors for injecting fuel directly into combustion chambers to produce stratified charge combustion by injecting fuel during a compression stroke in a low-speed, low-load range, for example, to thereby accomplish extremely lean mixture combustion.
In this type of engine, it is impossible to achieve sufficient emission-cleaning performance with respect to nitrogen oxides (NOx) under lean burn operating conditions by using an ordinary three-way catalyst alone, which is a catalyst having high performance to convert hydrocarbons (HC), carbon monoxide (CO) and NOx at about the stoichiometric air-fuel ratio, as an emission-cleaning converter. Therefore, as shown in the aforementioned Publication, the engine is provided with a lean NOx catalyst which adsorbs NOx in an oxygen-rich atmosphere and releases and reduces NOx in an atmosphere where oxygen concentration has decreased. If the amount of NOx adsorbed by the lean NOx catalyst has increased under the lean burn operating conditions when the lean NOx catalyst of this kind is being used, the fuel is injected not only for primary combustion but an additional amount of fuel is injected during an expansion stroke to lower the air-fuel ratio and generate CO for accelerating release and reduction of NOx as shown in the aforementioned Publication, for example.
In the aforementioned engine which performs conventional lean burn operation, it is necessary to provide the lean NOx catalyst in an exhaust passage to offer NOx-converting performance during the lean burn operating conditions. This type of engine also requires the three-way catalyst for cleaning emissions in such engine operating ranges as a high-load range in which the engine is operated at the stoichiometric air-fuel ratio. Thus, the three-way catalyst is provided together with the lean NOx catalyst in the exhaust passage. The lean NOx catalyst, which needs to have a relatively large capacity to provide a capability to adsorb some amount of NOx, is expensive compared to the three-way catalyst, so that the provision of the lean NOx catalyst is disadvantageous from the viewpoint of product cost.
Furthermore, it is necessary to temporarily lower the air-fuel ratio by supplying an additional amount of fuel to accelerate release and reduction of NOx at specific intervals of time when the amount of NOx adsorbed increases as stated above in order to maintain the converting performance of the lean NOx catalyst. This occurs rather frequently, resulting in deterioration of fuel economy improvement effects offered by lean burn operation.
In addition, the lean NOx catalyst is susceptible to poisoning by sulfurization if the used fuel contains high sulfur content. The lean NOx catalyst should therefore be subjected to regeneration treatment, such as catalyst heating and feeding of a reducing agent, to prevent this sulfur-poisoning problem. This regeneration treatment of the lean NOx catalyst is likely to jeopardize the fuel economy improvement effect and reduce its durability.
Under these circumstances, the inventors of the present invention previously filed Japanese Patent Application No. 2002-024548, which disclosed a technique concerning a control device for a spark-ignition engine capable of providing improved emission-cleaning performance by simply using a three-way catalyst, without the need of a lean NOx catalyst, while maintaining the fuel economy improvement effect offered by lean burn operation.